1. Field of the Invention
The present invention relates to a transmission circuit for use in communication apparatuses, such as mobile phones, wireless LANs and the like. More particularly, the present invention relates to a transmission circuit capable of low power consumption and broad-band operation.
2. Description of the Background Art
Recently, mobile communication systems (e.g., mobile phones, wireless LANs, etc.) use broad-band modulated signals for high-speed data transmission. Therefore, communication apparatuses provided in terminals need to operate in a broad band and with low power consumption. Communication apparatuses consume much power to amplify the output power of transmitted radio wave. Therefore, such communication apparatuses require a transmission circuit which can operate in a broad band and amplify output power with low power consumption.
Conventionally, polar modulation is known as a technique for operating a transmission circuit in a communication apparatus. Polar modulation is also called EER (Envelope Elimination and Restoration), in which a signal is separated into amplitude and angle components, the components are separately amplified/modulated in respective modulation sections, and the modulated amplitude components and the modulated angle components are combined. A transmission circuit employing polar modulation is disclosed in, for example, F. H. Raab, “High-efficiency L-band Kahn-technique transmitter”, 1998 IEEE MTT-S Int. Microwave Symp. Dig. Hereinafter, a conventional transmission circuit employing polar modulation will be described with reference to the accompanying drawings.
FIG. 24 is a block diagram showing an exemplary structure of the conventional transmission circuit employing polar modulation. Referring to FIG. 24, the transmission circuit comprises a data generation section 1901, an angle modulation section 1902, an amplitude modulation section 1903, a voltage control section 1904, and an output terminal 1905. The data generation section 1901 is connected via the angle modulation section 1902 to the gate or base of the amplitude modulation section 1903, and is also connected via the voltage control section 1904 to the drain or collector of the amplitude modulation section 1903. The output of the amplitude modulation section 1903 is connected to the output terminal 1905.
In the above-described transmission circuit, the data generation section 1901 generates a signal containing an amplitude data component (hereinafter referred to as an amplitude signal) and a signal containing a phase data component (hereinafter referred to as a phase signal), based on data to be transmitted. The amplitude signal is input to the voltage control section 1904. The voltage control section 1904 amplifies the input amplitude signal, and outputs the resultant signal to the amplitude modulation section 1903. In other words, the voltage control section 1904 supplies a voltage which has been controlled using the amplitude signal to the amplitude modulation section 1903.
The phase signal is input to the angle modulation section 1902. The angle modulation section 1902 angle-modulates the input phase signal and outputs the resultant angle-modulated wave signal. The amplitude modulation section 1903 amplitude-modulates the input angle-modulated wave signal based on the voltage supplied from the voltage control section 1904, and outputs the resultant modulated wave signal. Thus, the output terminal 1905 outputs the modulated wave signal.
Note that detailed structures of the amplitude modulation section 1903 and the voltage control section 1904 will be described with reference to FIGS. 25 to 27.
Firstly, a structure of the voltage control section 1904 will be described.
For example, a series regulator or a switching regulator can be applied as the voltage control section 1904.
Firstly, a voltage control section employing a series regulator will be described.
FIG. 25 is a block diagram showing an exemplary structure of the voltage control section employing a series regulator. Referring to FIG. 25, the voltage control section 1904 comprises an input terminal 2001, a power source 2002, an output terminal 2003, a comparison section 2004, and a transistor 2005.
In FIG. 25, the input terminal 2001 is connected to the data generation section 1901. The amplitude signal is input via the input terminal 2001 to the data generation section 1901. The amplitude signal received through the input terminal 2001 is input via the comparison section 2004 to the gate or base of the transistor 2005. In other words, a voltage which has been controlled based on the amplitude signal is applied to the gate or base of the transistor 2005. The drain or collector of the transistor 2005 is connected to the power source 2002, which applies a voltage thereto. Therefore, an amplified amplitude signal is output from the source or emitter of the transistor 2005. The amplitude signal amplified by the transistor 2005 is input via the output terminal 2003 to the drain or collector of the amplitude modulation section 1903. In other words, a voltage which has been controlled using the amplitude signal is applied to the drain or collector of the amplitude modulation section 1903.
In the voltage control section 1904 of FIG. 25, a signal output from the source or emitter of the transistor 2005 is fed back to the comparison section 2004 so that a stable output voltage can be obtained.
The voltage control section employing the series regulator has lower power efficiency (larger power consumption) than that of a voltage control section employing a switching regulator, but is known to be capable of operating in a broad band.
Next, the voltage control section employing a switching regulator will be described.
FIG. 26 is a block diagram showing an exemplary structure of the voltage control section employing a switching regulator. The voltage control section 1904 comprises a pulse conversion section 2101, an amplifier 2102, a lowpass filter 2103, an input terminal 2104, and an output terminal 2105.
Referring to FIG. 26, the input terminal 2104 is connected to the data generation section 1901. The amplitude signal is input via the input terminal 2104 to the data generation section 1901. The amplitude signal received through the input terminal 2104 is converted to a pulse signal by the pulse conversion section 2101. The pulse conversion section 2101 performs conversion using, for example, PWM, delta-sigma modulation or the like. The pulse signal is amplified by the amplifier 2102, and is transferred to the lowpass filter 2103. Note that the amplifier 2102 may be, for example, a class-D or -S amplifier for the purpose of highly efficient signal amplification.
A spurious signal having a clock frequency which occurs in the pulse generation is removed from the amplified pulse signal by the lowpass filter 2103 before the amplified pulse signal is output from the output terminal 2105. The signal output from the output terminal 2105 is input to the drain or collector of the amplitude modulation section 1903. In other words, a voltage whose output level has been controlled using the amplitude signal is applied to the drain or collector of the amplitude modulation section 1903. Note that the voltage control section 1904 may feed an output of the lowpass filter 2103 back to the pulse conversion section 2101.
The voltage control section employing the switching regulator does not operate in a broad band better than the voltage control section employing the series regulator, but is known to have higher power efficiency (lower power consumption).
Next, the amplitude modulation section 1903 will be described with reference to the drawings.
FIG. 27 is a block diagram showing an exemplary structure of the amplitude modulation section 1903 of FIG. 24. The amplitude modulation section 1903 comprises an input terminal 2201, an input terminal 2205, an output terminal 2202, a transistor 2203, a power source terminal 2204, a matching circuit 2206, a matching circuit 2207, a bias circuit 2208, and a bias circuit 2209.
Referring to FIG. 27, the input terminal 2201 is connected to the angle modulation section 1902. The angle-modulated wave signal is input via the input terminal 2201 to the angle modulation section 1902. The input terminal 2201 is also connected via the matching circuit 2206 to the gate or base of the transistor 2203. A DC voltage is applied to the power source terminal 2204. The input terminal 2205 is connected to the voltage control section 1904. The amplified amplitude signal is input via the input terminal 2205 to the voltage control section 1904. The input terminal 2205 is also connected via the bias circuit 2209 to the drain or collector of the transistor 2203. The output terminal 2202 is connected to the output terminal 1905.
In other words, the angle-modulated wave signal is input to the gate or base of the transistor 2203, and a voltage which has been controlled using the amplitude signal is applied to the drain or collector of the transistor 2203. The transistor 2203 amplitude-modulates the angle-modulated wave signal using the voltage which has been controlled using the amplitude signal, and outputs the resultant modulated wave signal. The modulated wave signal is output via the matching circuit 2207 to the output terminal 2202.
Note that the matching circuit 2206, the matching circuit 2207, the bias circuit 2208, and the bias circuit 2209 are provided in general amplitude modulation sections and will not be explained in detail.
A voltage control section employing a series regulator, such as that of FIG. 25, can operate in a broad band. However, the voltage control section uses the transistor 2005 as a variable resistance, and therefore, its power loss is large when an output voltage of the output terminal 2003 is small. As a result, a transmission circuit which comprises a voltage control section employing a series regulator, such as that of FIG. 25, has a large total of power consumption.
A voltage control section employing a switching regulator, such as that of FIG. 26, has high power efficiency, but does not operate in a broad band satisfactorily for the following reason. In general, a voltage control section needs to operate the pulse conversion section 2101 at a frequency 10 or more times than a signal band. However, it is difficult for the pulse conversion section 2101 to operate in such a broad band. In addition, when the voltage control section forces the pulse conversion section 2101 to operate in a broad band, power consumption is increased. As a result, a transmission circuit which comprises a voltage control section employing a switching regulator, such as that of FIG. 26, has difficulty in operating in a broad band, and when the voltage control section is forced to operate in a broad band, total power consumption is increased.
High-speed communication is keenly required for recent communication apparatuses. Therefore, transmission circuits which are applied to current communication apparatuses are composed of a voltage control section employing a series regulator, such as that of FIG. 25, which can operate in a broad band. As a result, transmission circuits capable of broad-band operation disadvantageously have high power consumption.